Put the Tape On Before the Cracks Show Up

The AV-8B Harrier engine inlet ring issues that have been readiness degraders for the jump jet since 1983 have been solved with a simple fix that provides a cost benefit and maintenance avoidance for the fleet.

Dr. Dave Barrett thought he had a solution based on existing technology and experience gained from research completed by NAWCAD engineers more than a decade ago. But Barrett, airframe technology team lead, knew he had a hard sell on his hands when he suggested that the only thing needed to stop the out-of-control vibrations and eventual sonic-induced fatigue in the Harrier’s engine inlet ring was a little damping.

The Harrier’s inlet ring, an aluminum sheet-metal duct-like structure running from the frame of the aircraft to the face of the engine, was experiencing excessive vibration, causing the inlet ring skin, fasteners and stiffeners to crack.

Looks simple, in retrospect...

"The engine was causing the structure to resonate," said Troy Hullander, aircraft structural integrity manager for the AV-8B Weapons System (PMA-257). "Basically at a critical point in the engine operation, it would make the structure 'sing.' It would spike up and increase the vibrations in the structure to the point where we would induce cycles at an extremely rapid rate [and] causing widespread cracking. Originally [the cracks] started at just the stiffeners on the structure."

An attempt at a fix by the original Harrier manufacturer, McDonnell-Douglas Aircraft Corporation, in 1983 during the AV-8B’s full-scale production program, was handled by adding more stiffeners and increasing the ring’s wall thickness.

As the fleet Harriers’ flight hours increased, the engine inlet cracking returned. The structure would crack at the engine inlet stiffeners, fasteners and skins. As the cracking of these components became a foreign object damage risk, the policy was to repair the cracks as they were found. This issue quickly became a high maintenance cost and fleet-readiness degrader.

A "tiger team" was assembled to attack the issues with the Harrier engine inlet ring. The tiger team included participants from the AV-8B Fleet Support Team and the Structures, Dynamics, Structural Technology, and Materials and Processing groups within NAVAIR and Boeing, which acquired the McDonnell-Douglas Aircraft Corporation and the Harrier program in 1997.

"We came up with 15 different possible fixes," Hullander said, "from the easy to the absolutely ridiculous. Then we started down-selecting. That’s when I started getting educated from Dave about damping systems."

The proposed solution that came out of the tiger team wasn’t damping, though – at least not at first.

The team took the classical approach to solve the cracking issue in the engine inlet by stiffening the structure. A modification concept was developed that increased the stiffness of the structure and showed promise. However, to install the modification would have required the removal of over 250 fasteners and a majority of the stiffeners, and replaced them with stronger parts. A concern arose within the team that the amount of work that had to be performed to the engine inlet during modification might cause more damage to the structure than the vibratory loads of the environment.

'Moving' the problem isn't the same as 'fixing' it.

But Barrett and Hullander both said that when complex systems are subjected to vibratory loads, the addition of more material is not necessarily beneficial and can "chase" the cracking problem to a new location.

"With the results of Dave’s [in-lab independent research] studies [on damping] we convinced the program office and they invested a small amount of money for us to do some design and verification work," Hullander said.

John Hill, an engineer from NAVAIR Depot North Island on rotation here at Pax River, set up a team with Jennifer Elmore of the Airframe Technology Team, and Rimi Rivera and Andy Guy from the Materials Division. They set up and conducted durability studies to make sure the configuration the team chose would, once stuck, stay stuck.

Wriley Gay of NAVAIR Depot Cherry Point provided an inlet ring and fuselage hulk for test. With the help of Doug Walling, also of Cherry Point, and through an existing Air Force contract, they sent the test articles to the University of Dayton Research Institute to come up with the design and perform the initial testing for validation and verification.

The Dayton Institute came up with a design that involved adhering the damping tile to the inlet ring with a visco-elastic tape. The damping material converts the kinetic energy of the structure to heat, and thereby reduces the vibration level.

"We went out and modified two aircraft," Hullander said. "One aircraft now has 120 hours on it. It went over to Iraq. One hundred twenty hours on it and [the damping tile] stuck."

Hearing that news for the first time, Barrett said, "That’s a pretty good demo." And because of the success of the demo, Hullander said, they have been given the go-ahead to incorporate the damping tiles on the rest of the fleet, which involves Navy Harriers and Harriers from four other countries as well.

"Based on the success of this program our division is pursuing another engineering effort," Barrett said. He has been helping the S-3 Viking program folks with an inlet problem the Viking is experiencing.

The Harrier’s engine inlet ring success story is an example of what the national NAVAIR team can do, Barrett said. One small investment made in the late 1980s at the Warminster Independent Research program, headed by Dr. Asha Varma, is paying great dividends as it already saved one project and four countries millions of dollars. And, more programs are planning to use damping as their cost-saving fix.

"The small investment made will give us immense returns on the investment, and also reduce the readiness problems we have with the fleet," Hullander.

[Thanks to Jim Jenkins, NAS Patuxent River Public Affairs Department --ed.]

FMI: www.nawcad.navy.mil